Improve pyright verification of third-party test cases in CI (#9650)

Co-authored-by: Avasam <samuel.06@hotmail.com>

pyrightconfig.testcases.json

@@ -3,6 +3,7 @@
     "typeshedPath": ".",
     "include": [
         "test_cases",
+        "stubs/**/@tests/test_cases"
     ],
     "typeCheckingMode": "strict",
     // Using unspecific "type ignore" comments in test_cases.